At the periphery of a nuclear power plant, spent nuclear fuel reprocessing facilities, and the like, installed are dose rate monitoring devices which can provide a wide range of dose rate measurements (refer to Patent Documents 1-6, for example). The dose rate measurements, ranging from a naturally occurring radiation level during a peace time, to a high radiation level at the time of an accident, are usually covered by installing two dose rate monitoring devices side by side, each of which is equipped with a radiation detector of a different kind. In this configuration, one of the dose rate monitoring devices which are installed side by side, may become an obstacle to the other dose rate monitoring device, with respect to the incoming radioactive rays from the space around. Thereby, measurement outcomes are affected by the side by side installation of the dose rate monitoring devices in no small part. It has been desired to provide a dose rate monitoring device which enables measurements ready for a wide range of the dose rate in a single body, also from the viewpoints of the down-sizing, cost reduction, and space saving of the entire monitoring device.
In the dose rate monitoring devices according to Patent Document 1 and Patent Document 2, a detector for a low range and a detector for a high range are arranged in a single detecting element section of the monitoring device, as a tactic way for solving these issues. The detector for the low range and the detector for the high range employ a scintillation detector and a semiconductor detector, respectively. The measurement range of the detectors is switched to output the measurement outcomes. In both Patent Documents, a NaI (Tl) scintillation detector which uses a thallium activation sodium iodide scintillator has been employed as a detector for the low range.
When a dose rate monitoring device employs a different radiation detector, the dose rate monitoring device will show different energy characteristics. Accordingly, a level difference arises in the reading of the dose rate monitoring device, when switching between the low range and the high range is performed for outputting the dose rate. The dose rate monitoring device according to Patent Document 1 measures the wave height spectrum of a detector output pulse received from at least one of the detectors, and estimates the energy of an incident radioactive ray, in order to reduce the level difference raised by this range switching. In the upper and lower dose rate fields including a switching point, a level difference of great size which arises at the switching point has been cancelled by uniting one of the energy characteristics at the dose rate fields with the other energy characteristic.
The sensitivity of radiation detectors is dependent on the incident direction of a radioactive ray. In the dose rate monitoring device according to Patent Document 2, a single semiconductor detector is provided on the head side of the cylindrical scintillator of a scintillation detector, and further two semiconductor detectors are provided on the side planes of the cylindrical scintillator, with 180 degrees apart in a circle. By employing the configuration of the semiconductor detectors mentioned above, the level difference due to the direction dependence of a detector for the low range and a detector for the high range has been reduced at the time of the range switching.